The present invention relates to a yttria containing water-base refractory slurry which is not subject to premature gelation and which can be used to form defect free molds and castings.
Yttria (yttrium oxide, Y.sub.2 O.sub.3), because of its refractoriness and chemical inertness, is a very desirable refractory for use as a face coat on molds or a coating on cores for use in casting reactive metals. This is because reactive metals or alloys tend to react with many known molds in such a manner as to form defective castings.
Colloidal silica is a very desirable and widely used binder for investment casting molds. The colloidal silicas (SiO.sub.2) usually used on a commerical basis have a silica content of approximately 30 percent. These known colloidal silicas are stabilized by an alkali (usually sodium oxide), and have an average silica particle size of either 7 or 14 mu (millimicrons). Colloidal silica is relatively inexpensive, stable, possesses excellent room temperature bonding characteristics, provides continuous bonding during all stages of the process, does not present a fire hazard and does not involve the use of organic solvents.
It would appear obvious to use yttria powder as part, or all, of the face coat refractory along with colloidal silica binder in the making of molds and/or cores for use with reactive metals. This is especially true for investment casting molds which are commonly made from colloidal silica. However, previous attempts to do this have been unsuccessful.
The problems involved in attempting to use yttria with colloidal silica have been well documented by Lassow et al. in U.S. Pat. No. 4,703,806 issued Nov. 3, 1987 and entitled "Ceramic Shell Mold Facecoat and Core Coating Systems for Reactive Casting of Reactive Metals". This patent reports the efforts of various investigators who were unsuccessful in using colloidal silica, as well as other water base refractory binders, with yttria powder. Some of these efforts are reported in the following three paragraphs from bottom of column 1 and the top of column 2 of U.S. Pat. No. 4,703,806:
For a number of years, yttria (Y.sub.2 O.sub.3) has been investigated as a possible mold facecoat material because of its low reactivity with respect to titanium. To make application of yttria economical, investigators have tried yttria-based slurries. Heretofore, however, investigators have been unsuccessful in using yttria-based slurries as mold facecoat materials in the fabrication of molds for casting reactive metals.
For example in 1976, Schuyler et al. reported the results of tests using fine particle yttria dispersed in colloidal potassium silicate solution to which coarse yttria has been added as a mold facecoat material. D. R. Schuyler, et al., "Development of Titanium Alloy Casting Technology," AFML-TR-76-80, Aug. 1976, pp. 275-279. The molds made with this facecoat material were not satisfactory. Schuyler et al. reported that "the facecoat was not as smooth as normal for the standard foundry system. Pores and pits were present, and the stucco showed through in many places." Schuyler et al. also tried a slurry containing yttria, titania and colloidal silica. Schuyler et al. found that with this system the facecoat surface was even more highly pitted.
It is particularly relevant to note that U.S. Pat. No. 4,703,806 teaches that a mold facecoat composition which comprises yttria powder and aqueous colloidal silica binder results in slurries which exhibit rapid and premature gelation. These slurries result in mold facecoats which tend to crack and/or spall during mold firing. As a result, U.S. Pat. No. 4,703,806 proposes to solve the problems resulting from using yttria powder with aqueous colloidal silica binder by using yttria with a non-aqueous binder. Ethyl silicate is suggested by the patent as being a preferred binder. However, other non-aqueous binders are also disclosed. In addition, U.S. Pat. No. 4,578,487 issued March 25, 1986 to Barfurth et al. and entitled "Binding Agents Containing Titanic Acid Esters for the Preparation of Coating Compositions and Refractory Bodies, and a Method for Preparation of These Binding Agents" suggests the use of a chelated organic titanium compound as a binder. This patent indicates that the chelated organic titanium compound can be used as a binder with yttria.
All of these non-aqueous refractory binders are more expensive than an aqueous based refractory binder. In addition, the non-aqueous binders suggested by the aforementioned prior art patents present fire and environmental hazards. The slurries which are made by using these non-aqueous refractory binders present stability problems since the slurries are sensitive to moisture which can be picked up from the atmosphere. In addition, slurries which have non-aqueous refractory binders have poor dipping/draining characteristics which tend to result in poor, non-uniform coatings.